1. Field of the Invention
This invention is a method of producing optical fibers with improved cross sectional circularity.
2. Description of the Prior Art
The significance of the role to be played by optical fibers in information transmission systems is no longer in dispute. The emphasis of research and development programs in this field has shifted from that of proving practicality to one of improving transmission efficiency. An active area, which has been particularly fruitful in yielding such improvements, involves the reduction of losses in optical fibers so that they may be used for long distance transmission. The lower the optical losses in such fibers the less frequent the need for multiple optical repeaters and, consequently, the cheaper the cost of the total system.
Two fabrication processes are found to yield particularly pure fibers of loss as low as 2 db/km. One process is commonly referred to as the soot deposition technique. The other is known as the modified chemical vapor deposition technique. In the soot deposition process, discussed in U.S. Pat. Nos. 3,826,560 and 3,823,195, glass particulate matter is formed in a hydrolysis burner and deposited on a starting member such as a glass rod. Additional layers of glass of possibly varying composition, the last suitable for a cladding, are deposited on the rod and the structure is then consolidated into a transparent glass by heating in an inert environment. Subsequent to this consolidation, the starting member may be removed, thereby forming a hollow cylindrical glass structure which may be drawn into a fiber.
In the modified chemical vapor deposition process, discussed in a commonly-assigned application Ser. No. 444,705, glass precursor vapors are directed through the center of a hollow glass cylinder which is heated to sufficiently high temperatures so as to initiate a homogenous reaction within the glass cylinder. In the course of this reaction, glass particulate matter is formed, deposited on the glass cylinder, and subsequently fused onto the cylinder and into a glass by the passage of the source of heat, which periodically traverses the cylinder. The starting glass cylinder may be composed of a material appropriate for use as a cladding in the fiber.
In both of these techniques, a hollow glass cylinder may be formed. Ordinarily the cylinder has at least two compositional regions. The interior region will ultimately form the core of the optical fiber through which the optical radiation will pass. The exterior region forms the cladding for the optical fiber. The remaining critical step involves pulling this relatively large diameter (5 to 25 mm) cylindrical "preform" into a relatively small diameter (5 to 100 microns) fiber. Prior to pulling the preform into a fiber, the preform is usually collapsed to a smaller diameter, or preferably into a solid cylindrical mass.
During both preform fabrication and preform collapse, noncircularities are introduced into the otherwise circular preform cross section. If these asymmetries are not removed before pulling the preform into a fiber, they will be reflected in the cross section of the resultant fiber yielding a noncircular optical waveguide cross section. Such asymmetrical fibers are difficult to splice to other optical fibers with different cross-sectional properties, and may yield a fiber with degraded pulse dispersion properties.
The asymmetry which develops during collapse is pronounced in multi-layered preforms. Such preforms have layers of material of decreasing index of refraction as the cross section is traversed radially from the core to the cladding. These gradations yield improved transmission characteristics. However, if the interior core layers are significantly less viscous than the outer cladding layers, then the interior layers will loss much of their structural integrity during the heating to which the preform is exposed for purposes of collapse. These interior layers will then provide little support for the more viscous outer layers. This only exacerbates the tendency toward noncircular collapse.
In simple two-layered preforms, the tendency toward noncircular collapse may be especially severe. Such preforms are used to yield a fiber with high numerical aperture and, hence, a larger acceptance or entry angle for introducing radiation into the fiber. In such fibers, the core must have a significantly larger index of refraction than the cladding. Silica, highly doped with germania, provides such a core but, in addition, has much lower viscosity than the silica cladding. Significant noncircularities develop in the course of fabricating such a preform. Similar difficulties appear when other compositions are used. The severity of the noncircularities depends on the diameter of the preform, the materials utilized, and the fabrication parameters chosen.
One method to effect more symmetrical and circular cross sections involves collapsing the preform while passing a gas stream through the center of the preform -- a dynamic process. The effectiveness of this technique was found to be limited. In addition it appears that the large flow of gas through the center of the tubular preform at collapsing temperatures results in dopant loss from the core material, thereby degrading the transmission characteristics of the resultant fiber.